Method of generating multiple current sources from a single reference resistor

ABSTRACT

A differential voltage controlled current source generating one or more output currents is based upon a single external resistor. The differential voltage controlled current source may generate an output current that is proportional to a received differential voltage and a bias current with the use of a single external resistor. The technique may be used to generate multiple accurate and process independent current sources. The current sources may be a zero temperature coefficient (ZTC) current, a proportional to absolute temperature (PTAT) current, or an inversely proportional to absolute temperature (NTAT) current. The output of the current sources may be inversely proportional to the resistance of the external resistor.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent application Ser. No. 13/036,479, entitled “METHOD OF GENERATING MULTIPLE CURRENT SOURCES FROM A SINGLE REFERENCE RESISTOR,” which was filed on Feb. 28, 2011, and which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The embodiments disclosed herein are related to accurately generating currents in an integrated circuit. In particular, the embodiments disclosed herein are related to generation of one or more reference currents in an integrated circuit from a single external resistor.

BACKGROUND

As the need to reduce current in transceivers, radio frequency amplifiers, and other integrated circuits increases, the need to more accurately control currents of the integrated circuits also increases. In addition, an integrated circuit may require multiple current sources that have different temperature coefficients. As an example, a zero temperature coefficient (ZTC) current source may be used to develop a bias current. In some applications, a proportional to absolute temperature (PTAT) current source or an inversely proportional to absolute temperature (NTAT) current source may be useful to compensate for temperature drift. Furthermore, as power consumption requirements become more restrictive, there may be times in which a particular application needs to accurately set a bias current based upon an external reference element. For example, there may be a desire to set a bias current based upon one or more precision resistors coupled to the integrated circuit.

Even so, process drift and batch-to-batch differences may reduce the accuracy of internally generated currents and thereby reduce the yield of these integrated circuits. Thus, there is a need for a circuit and technique to generate process independent and batch independent current sources for integrated circuit applications to improve manufacturing yields.

SUMMARY

Embodiments disclosed in the detailed description relate to a differential voltage controlled current source generating one or more output currents based upon a single external resistor. A differential voltage controlled current source may generate multiple currents based upon a single external resistor. The differential voltage controlled current source may generate an output current that is proportional to a received differential voltage and a bias current with the use of a single external resistor. The technique may be used to generate multiple accurate and process independent current sources. The current sources may be a zero temperature coefficient (ZTC) current, a proportional to absolute temperature (PTAT) current, or an inversely proportional to absolute temperature (NTAT) current. The output of the current sources may be inversely proportional to the resistance of the external resistor.

The embodiments described in the detailed description may further relate to a technique for generating multiple accurate and process independent ZTC, PTAT, and NTAT currents from a single external accurate resistor. For an exemplary n-type semiconductor, the external resistor is used to generate a current that is inversely proportional to the product of the mobility of an electron in an n-type semiconductor material (μ_(n)) and the capacitance of an oxide layer (C_(ox)) for a metal on semiconductor transistor, μ_(n)C_(ox). The current that is inversely proportional to μ_(n)C_(ox) biases a differential pair. As a result, the transconductance, Gm, of the differential pair is a constant. The constant Gm differential pair may then be driven by one of a ZTC reference voltage, a PTAT reference voltage, or an NTAT reference voltage. A subtractor circuit may be used to subtract half of the bias current of the differential pair to yield one of a ZTC, PTAT, or NTAT current.

An exemplary embodiment of a semiconductor circuit configured to generate a current proportional to a differential voltage includes a bias circuit coupled to a differential pair circuit. A first bias current through the bias circuit is set by a resistance of an external resistor. The bias circuit provides a first bias voltage based upon the first bias current. The differential pair circuit includes a first leg corresponding to a first voltage input and having a first leg current, a second leg corresponding to a second voltage input and having a second leg current, and a current source. The current source of the differential pair circuit provides a second bias current to the differential pair circuit based upon the first bias voltage. The current subtractor circuit is coupled to the second leg of the differential pair circuit and the bias circuit. The current subtractor circuit may be configured to generate a load current in the output diode load substantially equal to the second leg current minus one-half of the second bias current. An output current source is coupled to the output diode load and configured to mirror the load current. The output current source may generate an output current that is proportional to a voltage difference between the second voltage input and the first voltage input.

Another exemplary integrated circuit includes a bias circuit configured to generate a first bias current referenced to a resistance, R, of an external resistor. A first transistor and a second transistor may be configured to form a differential pair circuit, where the differential pair circuit includes a second bias current source configured to mirror the first bias current to generate a second bias current. The first transistor of the differential pair receives a first input voltage. The second transistor of the differential pair receives a second input voltage. A third transistor is configured to mirror the drain current of the second transistor. A fourth transistor is coupled to the third transistor and configured to have a drain current substantially equal to one-half of the second bias current. A fifth transistor is coupled to the third transistor and fourth transistor, where the fifth transistor is configured to have a drain current substantially equal to a difference between the drain current of the third transistor and the drain current of the fourth transistor. A sixth transistor is configured to mirror the drain current of the fifth transistor, where a drain current of the sixth transistor is proportional to a difference between the first input voltage and the second input voltage divided by the resistance, R, of the external resistor.

Another exemplary embodiment of a semiconductor circuit configured to generate a current proportional to a differential voltage includes a bias circuit, a differential pair circuit, a current subtractor and an output current source. A first bias current through the bias circuit is set by a resistance of an external resistor. The bias circuit provides a first bias voltage based upon the first bias current. The differential pair circuit may include a first leg corresponding to a first voltage input and have a first leg current, a second leg corresponding to a second voltage input and have a second leg current, and a current source. The current source may provide a second bias current to the differential pair circuit based upon the first bias voltage. The current subtractor circuit may include an output diode load, where the current subtractor circuit is coupled to the second leg of the differential pair circuit and the bias circuit. The current subtractor circuit may be configured to generate a load current in the output diode load substantially equal to the first leg current less one-half of the second bias current. The output current source may be configured to mirror the load current. The output current source may produce an output current that is proportional to a voltage difference between the second voltage input and the first voltage input.

Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIG. 1 depicts an exemplary embodiment of a differential voltage controlled current source referenced to one external resistor.

FIG. 2 depicts an exemplary current source circuit to provide multiple currents referenced to one external resistor.

FIG. 3 depicts an exemplary embodiment of a differential voltage controlled current source referenced to one external resistor.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

Embodiments disclosed herein relate to a differential voltage controlled current source generating one or more output currents based upon a single external resistor. A differential voltage controlled current source may generate multiple currents based upon a single external resistor. The differential voltage controlled current source may generate an output current that is proportional to a received differential voltage and a bias current with the use of a single external resistor. The technique may be used to generate multiple accurate and process independent current sources. The current sources may be a ZTC current, a PTAT current, or an NTAT current. The output of the current sources maybe inversely proportional to the resistance of the external resistor.

The embodiments described in the detailed description may further relate to a technique for generating multiple accurate and process independent zero temperature coefficient (ZTC), proportional to absolute temperature (PTAT), and inversely proportional to absolute temperature (NTAT) currents from a single external accurate resistor. For an exemplary n-type semiconductor, the external resistor is used to generate a current that is inversely proportional to the product of the mobility of an electron in an n-type semiconductor material (μ_(n)) and the capacitance of an oxide layer (C_(ox)) for a metal on semiconductor transistor, μ_(n)C_(ox). The current that is inversely proportional to μ_(n)C_(ox) biases a differential pair. As a result, the transconductance, Gm, of the differential pair is a constant. The constant Gm differential pair may then be driven by one of a ZTC reference voltage, a PTAT reference voltage, or an NTAT reference voltage. A subtractor circuit may be used to subtract half of the bias current of the differential pair to yield one of a ZTC, PTAT, or NTAT current.

FIG. 1 depicts a block diagram of an exemplary embodiment of a semiconductor device current source circuit 10 that includes a differential voltage controlled current source referenced to a single external resistor, R₁. FIG. 1 depicts a bias circuit 12 formed by transistors M₁, M_(2,) M₃, M₄, and an external precision resistor R₁ with a resistance R. The transistors M₄ and M₃ may be configured as current sources to provide current to the transistors M₂ and M₁, respectively. The source of the transistor M₄ and the source of transistor M₃ are each coupled to a supply voltage, V_(SUPPLY). The gates of the transistors M₄ and M₃ are coupled to the drain of transistor M₄ to form a first current mirror, where the current flowing through transistor M₄ is proportional to the current flowing through transistor M₃. The gate of transistor M₁ and the gate of transistor M₂ are coupled to the drain of transistor M₂ to form a second current mirror. The current through transistor M₂ is proportional to the current flowing through transistor M₁. The drain of transistor M₃ is coupled to the drain and gate of transistor M₂, which configures transistor M₂ to be a diode load that carries a bias current, I_(BIAS). The source of transistor M₂ is coupled to ground. The drain of transistor M₁ is coupled to the drain of transistor M₄. The source of M₁ is coupled to resistor R₁. The current flowing through the resistor R₁, combined with the gate to source voltage of transistor M₁, provides a gate bias voltage, V_(BIAS), on the gates of transistors M₁ and M₂. The bias current, I_(BIAS), generated through the transistor M₂ by the bias circuit, is given by equation (1).

$\begin{matrix} {I_{BIAS} = {\frac{2}{\mu_{n}C_{ox}R^{2}}\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}} & (1) \end{matrix}$

where (w/L)₁ is the ratio of the channel width to the channel length of the transistor M₁, and (w/L)₂ is the ratio of the channel width to the channel length of the transistor M₂.

FIG. 1 further depicts a differential pair circuit 14 including transistors M₅, M₆, M₇, M₈, and M₉. The differential pair circuit 14 includes a first leg, formed by the transistors M₅ and M₇, and a second leg, formed by the transistors M₆ and M₈. The sources of transistors M₅ and M₆ are each coupled to the supply voltage, V_(SUPPLY). The gate of transistor M₅ is coupled to the drain of transistor M₅ to form a diode current source for transistor M₇, which provides a first current, I₁, to the drain of transistor M₇. The gate of transistor M₆ is coupled to the drain of transistor M₆ to form a diode current source, which provides a second current, I₂, to the drain of transistor M₈. A bias current, I_(CC), for the differential pair circuit is developed by coupling the gate of the transistor M₉ to the bias voltage, V_(BIAS), at the gate of transistor M₂. The current flowing through transistor M₉ will be proportional to the bias current, I_(BIAS), passing through transistor M₂. The differential pair circuit includes a first input voltage, V₁, at the gate of transistor M₇ and a second input voltage, V₂, at the gate of transistor M₈.

The large signal transconductance of the transistor M₇, Gm₁, and the large signal transconductance of the transistor M₈, Gm₂, in the differential pair circuit 14 is described in equation (2). The drain current I_(d1) corresponds to the current flowing through the drain of the transistor M₇. The drain current I_(d2) corresponds to the current flowing through the transistor M₈. The ratio of channel width to channel length of transistors M₇ and M₈, (W/L), are the same. Because μ_(n)C_(ox) varies with temperature and process, the transconductances Gm₁ and Gm₂ of the differential pair circuit 14 may vary with process and temperature, as shown in equation (2).

$\begin{matrix} {{{Gm}_{1} = \sqrt{2I_{d\; 1}\mu_{n}{C_{ox}\left( \frac{W}{L} \right)}}}{{Gm}_{2} = \sqrt{2I_{d\; 2}\mu_{n}{C_{ox}\left( \frac{W}{L} \right)}}}} & (2) \end{matrix}$

However, the process and temperature variations of Gm₁ and Gm₂ may be made constant over process and temperature by configuring the transistor M₉ to mirror the current I_(BIAS) passing through transistor M₂. Accordingly, the transconductance, Gm, of the differential pair circuit 14 with the constant current source, I_(CC), set equal to the current I_(BIAS) is given by equation (3).

$\begin{matrix} {{Gm}_{i} = \sqrt{\frac{2}{R^{2}}\left( \frac{W}{L} \right)\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2\;}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1\;}}}} \right)^{2}}} & (3) \end{matrix}$

where Gm, is proportional to 1/R, as shown in equation (3.a).

$\begin{matrix} {{Gm}_{i} = {\frac{1}{R}\sqrt{2\left( \frac{W}{L} \right)\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}}} & \left( {3.a} \right) \end{matrix}$

Ignoring channel length/mobility modulation, when V₁=V₂, the drain currents, I_(d1) and I_(d2), in transistors M₇ and M₈, respectively, are equal and given by equation (4).

$\begin{matrix} {I_{d\; 1} = {I_{d\; 2} = {\frac{\mu_{n}C_{ox}}{2}\left( \frac{W}{L} \right)\left( {V_{{gs}_{i}} - V_{t}} \right)^{2}}}} & (4) \end{matrix}$

where Vgs_(i) is the gate to source voltage of transistors M₇ and M₈, V_(t) is the threshold voltage of transistors M₇ and M₈, and (W/L) is the channel width to channel length ratio of transistors M₇ and M₈.

By substitution, if V₂>V₁, the gate to source voltages of the transistors M₇ and M₈ are V_(gs2) and V_(gs1), respectively. The change in drain current through transistors M₇ and M₈ is given by equations (5), (6), respectively.

$\begin{matrix} {{\Delta \; I_{d\; 1}} = {{G_{m}\left( {V_{gs} - V_{{gs}\; 1}} \right)}\left\lbrack {1 + \frac{\left( {V_{{gs}\; 1} - V_{gs}} \right)}{2V_{dsat}}} \right\rbrack}} & (5) \\ {{{\Delta \; I_{d\; 2}} = {{G_{m}\left( {V_{{gs}\; 2} - V_{gs}} \right)}\left\lbrack {1 + \frac{\left( {V_{g\; s\; 2} - V_{gs}} \right)}{2V_{dsat}}} \right\rbrack}}{where}} & (6) \\ {V_{{dsat}_{i}} = \sqrt{\frac{2I_{d_{i}}}{\mu_{n}C_{ox}}\left( \frac{L}{W} \right)}} & (7) \end{matrix}$

Assuming that V_(dsat) is very large relative to (2V_(gs)−V_(gs2)−V_(gs1)), the difference of the currents in M₆ and M₇ is given by equation (8).

$\begin{matrix} \begin{matrix} {{\Delta \; I_{d}} = {{\Delta \; I_{d\; 1}} + {\Delta \; I_{d\; 2}}}} \\ {= {G_{m}\left( {V_{{gs}\; 2} - V_{{gs}\; 1}} \right)}} \\ {= {G_{m}\left( {V_{2} - V_{1}} \right)}} \end{matrix} & (8) \\ {{{With}\mspace{14mu} V_{dsat}}\operatorname{>>}\left( {{2V_{gs}} - V_{{gs}\; 2} - V_{{gs}\; 1}} \right)} & \; \end{matrix}$

Accordingly, the current I₂ through transistor Mg is given by equation (9).

$\begin{matrix} {I_{2} = {{{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)} + \frac{I_{BIAS}}{2}}} & (9) \end{matrix}$

The current I₁ through transistor M₇ is given by equation (10).

$\begin{matrix} {I_{1} = {{{Gm}\left( \frac{V_{1} - V_{2}}{2} \right)} + \frac{I_{BIAS}}{2}}} & (10) \end{matrix}$

Transistors M₁₀, M₁₁, and M₁₂ form a current subtractor circuit 16 having an output current I_(SUB), which passes through transistor M₁₂. The current passing through transistor M₁₁ is subtracted from the current passing through transistor M₁₀ to generate the output current, I_(sub), where the transistor M₁₂ is configured as a load diode by coupling the gate of the transistor M₁₂ to the drain of the transistor M₁₂. The source of the transistor M₁₂ is coupled to ground.

The gate of transistor M₁₀ is coupled to the gate of transistor M₆. The source of transistor M₁₀ is coupled to the supply voltage, V_(SUPPLY). The transistor M₁₀ is configured to mirror the current I₂, which passes through the drain of transistor M₆.

The gate of transistor M₁₁ is coupled to the gate of transistor M₂. The source of transistor M₁₁ is coupled to ground. The drain of transistor M₁₁ is coupled to the drain of the transistor M₁₀ and the drain of transistor M₁₂. The transistor M₁₁ is configured to mirror one-half of the current I_(BIAS) passing through M₂. Accordingly, the current passing through the drain of transistor M₁₂, I_(SUB), is equal to the difference of the drain current of transistor M₁₀ less the drain current of transistor M₁₁, as given by equation (11).

$\begin{matrix} {I_{SUB} = {{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)}} & (11) \end{matrix}$

The current passing through transistor M₁₂ may be mirrored by transistor M₁₃ to generate an output current, I_(OUT), as shown in equation (12).

$\begin{matrix} {I_{OUT} = {{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)}} & (12) \end{matrix}$

Equation (12) may also be re-written in terms of equation (3(a)), as shown in equation (12.a), where I_(OUT) is proportional to 1/R.

$\begin{matrix} {I_{OUT} = {\frac{\left( {V_{2} - V_{1}} \right)}{R}\sqrt{\left( \frac{1}{2} \right)\left( \frac{W}{L} \right)\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}}} & \left( {12.a} \right) \end{matrix}$

Because Gm is process and temperature independent, the output current, I_(OUT), passing through transistor M₁₃ is also process and temperature independent.

FIG. 2 depicts an exemplary embodiment of a p-type doped semiconductor device current source circuit 20, which operates in a similar manner as the current source circuit 10.

The current source 20 includes a bias circuit 22, a differential pair circuit 24, and a current subtractor circuit 26. The bias current circuit 22 includes transistors Q₁, Q₂, Q₃, and Q₄ configured to generate a bias current, I_(BIAS), through transistor Q₂. Similar to the bias circuit 12 of FIG. 1, the bias current I_(BIAS) passing through transistor Q₂ is set based upon the resistance of an external resistor R₂, which generates a bias voltage, V_(BIAS), at the gates of transistors Q₁ and Q₂. The transistors Q₃ and Q₄ are configured as current sources that are coupled to transistors Q₂ and Q₁, respectively. The gate of the transistor Q₃ is coupled to the drain of the transistor Q₃ and the gate of the transistor Q₄. The sources of the transistors Q₃ and Q₄ are coupled to ground. The drain of the transistor Q₃ is coupled to the drain of the transistor Q₂. The source of the transistor Q₂ is coupled to the supply voltage, V_(SUPPLY). The gates of the transistors Q₁ and Q₂ are both coupled to the drain of the transistor Q₁. The source of the transistor Q₁ is coupled to an external resistor R₂, which has a resistance R. Thus, similar to the bias circuit 10 of FIG. 1, the transistor Q₂ of FIG. 2 is configured to pass the bias current, I_(BIAS), as a function of the resistance, R, of the external resistor R₂, as shown in equation (13).

$\begin{matrix} {I_{BIAS} = {\frac{2}{\mu_{p}C_{ox}R^{2}}\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}} & (13) \end{matrix}$

where (w/L)₁ is the ratio of the channel width to the channel length of the transistor Q₁, where (w/L)₂ is the ratio of the channel width to the channel length of the transistor Q₂, and R is the resistance of the external resistor R₂.

Similar to the differential pair circuit 14 of FIG. 1, the differential pair circuit 24 of FIG. 2 includes a first leg and a second leg coupled to a constant current source formed by the transistor Q₉. The gate of the transistor Q₉ is coupled to the gates of the transistors Q₁ and Q₂. The source of the transistor Q₉ is coupled to the supply voltage, V_(SUPPLY). As a result, the current passing through the drain of the transistor Q₉ mirrors the current passing through the transistor Q₂.

The first leg of the differential pair includes transistors Q₅ and Q₇. The gate of transistor Q₅ is coupled to the drain of transistor Q₅. The source of the transistor Q₅ is coupled to ground. The drain of transistor Q₇ is coupled to the drain of Q₅, where the drain current of the transistor Q₇ is I₁. The source of the transistor Q₇ is coupled to the source of the transistor Q₈ and the drain of the transistor Q₉. Similarly, the second leg of the differential pair includes transistors Q₆ and Q₈. The gate of the transistor Q₆ is coupled to the drain of the transistor Q₆. The source of the transistor Q₆ is coupled to ground. The drain of the transistor Q₆ is coupled to the drain of the transistor Q₈, wherein the drain current of transistor Q₈ is I₂. The source of the transistor Q₈ is coupled to the source of the transistor Q₇ and the drain of the transistor Q₉. The differential pair circuit includes a first input voltage, V₁, at the gate of transistor Q₇ and a second input voltage, V₂, at the gate of transistor Q₈. Similar to the differential pair circuit 14 of FIG. 1, the differential pair circuit 24 of FIG. 2 is configured such that the current I₁ passing through the drain of the transistor Q₇ is given by equation (14).

$\begin{matrix} {I_{1} = {{{Gm}\left( \frac{V_{1} - V_{2}}{2} \right)} + \frac{I_{BIAS}}{2}}} & (14) \end{matrix}$

where the transconductance, Gm, of the differential pair circuit 24 with the bias current set equal to the I_(BIAS) is given by equation (15).

${Gm} = {\frac{1}{R}\sqrt{2\left( \frac{W}{L} \right)\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}}$

where (W/L) is the ratio of the channel width to channel length of the transistors Q₇ and Q₈, where (w/L)₂ is the ratio of the channel width to channel length of transistor Q₂, and where (w/L)₁ is the ratio of channel width to channel length of the transistor Q₁.

Similar to the current subtractor circuitry 16 of FIG. 1, the current subtractor circuitry 26 of FIG. 2 includes a transistor Q₁₁ configured to mirror the current of the transistor Q₂, where the transistor Q₁₁ is configured to pass a drain current of I_(BIAS)/2. The drain of the transistor Q₁₁ is coupled to the drain of the transistor Q₁₀, which is configured to mirror the current passing through the transistor Q₅. Accordingly, the current I_(SUB) passing through the drain of transistor Q₁₂ is equal to the difference of the drain current of transistor Q₁₁ less the drain current of transistor Q₁₀, as given by equation (16).

$\begin{matrix} {I_{SUB} = {{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)}} & (16) \end{matrix}$

The transistor Q₁₃ is coupled to the gate and drain of the transistor Q₁₂. The source of the transistor Q₁₃ is coupled to V_(SUPPLY). As a result, the current passing through transistor Q₁₂ may be mirrored by transistor Q₁₃ to generate an output current, I_(OUT), that is proportional to the current passing through the transistor Q₁₂, I_(SUB), as shown in equation (17).

$\begin{matrix} {I_{OUT} = {{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)}} & (17) \end{matrix}$

Similar to the current source circuit 10 of FIG. 1, because Gm is process and temperature independent, the output current, I_(OUT), passing through transistor Q₁₃ is also process and temperature independent.

FIG. 3 depicts an implementation of a current source generator 28 having a current source circuit 30. The current source circuit 30 may be implemented in either NMOS or PMOS, which correspond to the current source circuit 10 of FIG. 1 and the current source circuit 20 of FIG. 2, respectively. The current source circuit 30 functions and operates in a similar manner as the current source circuit 10 and the current source circuit 20, as described above, where the output current is given by equations (18) and (18.a).

$\begin{matrix} {{I_{OUT} = {{Gm}\left( \frac{V_{2} - V_{1}}{2} \right)}}{and}} & (18) \\ {I_{OUT} = {\frac{\left( {V_{2} - V_{1}} \right)}{R}\sqrt{\left( \frac{1}{2} \right)\left( \frac{W}{L} \right)\left( {\frac{1}{\sqrt{\left( \frac{w}{L} \right)_{2\;}}} - \frac{1}{\sqrt{\left( \frac{w}{L} \right)_{1}}}} \right)^{2}}}} & \left( {18.a} \right) \end{matrix}$

Similar to the current source circuit 10 of FIG. 1 and the current source circuit 20 of FIG. 20, the current source circuit 30 may include an output of a bias current, I_(BIAS), which may be provided as an output by mirroring the current passing through the transistor M₂ of FIG. 1 or the transistor Q₂ of FIG. 2, as depicted in FIG. 3.

The current source circuit 30 may include an external resistor port for receiving an external precision resistor R₃ that sets the bias current, I_(BIAS), of the current source circuit 30. The current source generator 28 may include a reference voltage generator 32. The reference voltage generator 32 may include a first reference voltage output, V_(OUT), and a second reference voltage output, V_(REF), where the first reference voltage output, V_(OUT), is greater than the second reference voltage output, V_(REF). The first reference voltage output, V_(OUT), of the reference voltage generator 32 may be coupled to the second input voltage, V₂, of the current source circuit 30. The second reference voltage output, V_(REF), of the reference voltage generator 32 may be coupled to the first input voltage, V₁ of the current source circuit 30.

The reference voltage generator 32 may generate various differential voltages depending upon the needs of a particular semiconductor circuit. As an example, the reference voltage generator 32 may be a band gap circuit, which provides a constant voltage over the temperature of the band gap circuit. Because the output current, I_(OUT), of the current source circuit 30 is proportional to the second input voltage, V₂, less the first input voltage, V₁, the output current I_(OUT), will maintain a constant value over temperature and process variations. In addition, the output current, I_(OUT), of the current source circuit 10 will be referenced back to the resistance, R, of the external precision resistor R₃.

As another example, the current source generator 30 may be configured to produce a proportional to absolute temperature current, I_(PTAT), by using a PTAT voltage circuit as the second reference voltage of the reference voltage generator 32, where the output current, I_(OUT), is referenced back to the resistance, R, of the external precision resistor R₃.

As another example, the current source circuit 30 may be used to generate an inversely proportional to absolute temperature current, I_(NTAT), by using a NTAT voltage circuit as the voltage reference circuit 32, where the output current, I_(OUT), is referenced back to the resistance, R, of the external precision resistor R₁.

In addition, the I_(BIAS) current may be provided as a second current output by mirroring the current passing through transistor M₂ of FIG. 1.

Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A semiconductor circuit configured to generate a current proportional to a differential voltage comprising: a bias circuit configured to generate a bias voltage; a differential pair circuit having a leg and being operable to receive a first voltage input and a second voltage input, wherein the differential pair circuit is configured to generate a bias current based on the bias voltage and the leg is configured to generate a leg current; a current subtractor circuit coupled to the differential pair circuit and the bias circuit wherein the current subtractor circuit is configured to generate a load current substantially equal to the leg current minus a proportion of the bias current; an output current source configured to mirror the load current, wherein the output current source produces an output current that is proportional to a voltage difference between the first voltage input and the second voltage input.
 2. The semiconductor circuit of claim 1 wherein the proportion of the bias current is one-half.
 3. The semiconductor circuit of claim 1 further comprising a band gap voltage circuit electrically coupled to the differential pair circuit, wherein the band gap voltage circuit is configured to provide a differential voltage output across the first voltage input and the second voltage input of the differential pair circuit; and wherein the output current source is substantially constant with respect to temperature over a temperature range of the band gap voltage circuit.
 4. The semiconductor circuit of claim 1 further comprising a proportional to absolute temperature voltage source circuit, wherein the proportional to absolute temperature voltage source circuit provides a differential voltage output across the first voltage input and the second voltage input of the differential pair circuit.
 5. The semiconductor circuit of claim 1 further comprising a proportional to absolute temperature voltage source circuit, wherein the proportional to absolute temperature voltage source circuit provides a differential voltage output across the first voltage input and the second voltage input of the differential pair circuit, and wherein the output current source is a proportional to absolute temperature current source over a temperature range of the proportional to absolute temperature voltage source circuit.
 6. The semiconductor circuit of claim 1 further comprising an inversely proportional to absolute temperature voltage source circuit, wherein the inversely proportional to absolute temperature voltage source circuit provides a differential voltage output across the first voltage input and the second voltage input of the differential pair circuit, and wherein the output current source is a proportional to absolute temperature current source over a temperature range of the proportional to absolute temperature voltage circuit.
 7. The semiconductor circuit of claim 1 wherein the semiconductor circuit is implemented in an NMOS process.
 8. The semiconductor circuit of claim 1 wherein the output current is proportional to the voltage difference between the second voltage input and the first voltage input divided by a resistance of a resistor.
 9. The semiconductor circuit of claim 1 wherein the differential pair circuit includes a large signal transconductance, wherein the large signal transconductance is inversely proportional to a resistance of a resistor.
 10. The semiconductor circuit of claim 1 further comprising a reference voltage generator including a first reference voltage output and a second reference voltage output; wherein the first reference voltage output is coupled to the first voltage input and the second reference voltage output is coupled to the second voltage input.
 11. An integrated circuit comprising: a bias circuit configured to generate a first bias current set by a resistance; a first transistor and a second transistor configured to form a differential pair circuit, wherein the first transistor receives a first input voltage and the second transistor receives a second input voltage and the differential pair circuit is configured to mirror the first bias current to generate a second bias current; a third transistor having a drain current, wherein the third transistor is configured to mirror a drain current of the second transistor; a fourth transistor coupled to the third transistor, the fourth transistor configured to mirror the second bias current, wherein the fourth transistor is configured to have a drain current substantially equal to a proportion of the second bias current; a fifth transistor coupled to the third transistor and the fourth transistor, wherein the fifth transistor is configured to have a drain current substantially equal to a difference between the drain current of the third transistor and the drain current of the fourth transistor; and a sixth transistor configured to mirror the drain current of the fifth transistor, wherein a drain current of the sixth transistor is proportional to a difference between the first input voltage and the second input voltage divided by the resistance.
 12. The integrated circuit of claim 11 wherein the drain current of the sixth transistor is proportional to the drain current of the fifth transistor.
 13. The integrated circuit of claim 11 wherein the second bias current is substantially equal to the first bias current.
 14. The integrated circuit of claim 11 wherein a resistor having the resistance is external to the integrated circuit.
 15. The integrated circuit of claim 11 wherein the proportion is equal to one-half.
 16. A semiconductor circuit configured to generate a current proportional to a differential voltage comprising: a bias circuit configured to generate a first bias current set by a resistance; a differential pair circuit operable to receive a first voltage input and a second voltage input and having a leg, wherein the differential pair circuit is configured to mirror the first bias current to generate a second bias current and the leg is configured to generate a leg current; a current subtractor circuit including an output diode load, wherein the current subtractor circuit is coupled to a second leg of the differential pair circuit and the bias circuit, and wherein the current subtractor circuit is configured to generate a load current substantially equal to the leg current minus a proportion of the second bias current; an output current source configured to mirror the load current, wherein the output current source produces an output current that is proportional to a voltage difference between the second voltage input and the first voltage input.
 17. The semiconductor circuit of claim 16 wherein the output current is a first output current, and further including a current mirror coupled to the bias circuit, wherein the current mirror is configured to mirror the first bias current to generate a second output current.
 18. The semiconductor circuit of claim 16 further comprising a band gap voltage circuit electrically coupled to the differential pair circuit, wherein the band gap voltage circuit is configured to provide a differential voltage output across the first voltage input and the second voltage input of the differential pair circuit; and wherein the output current source is substantially constant with respect to temperature over a temperature range of the band gap voltage circuit.
 19. The semiconductor circuit of claim 16 wherein the semiconductor circuit is implemented in a PMOS process.
 20. The semiconductor circuit of claim 16 wherein the proportion is equal to one-half. 